Rudder Location

[SuperPiper]

I sail a pocket-cruiser with a transom-mounted rudder. I am about to modify the pintle and gudgeon hardware to incorporate boarding handles/ladder (every component must have multiple functions to justify its weight, eh). This modification will move the rudder about 2-1/2" outboard of the transom.

However, I recently saw what I believe was a C&C 25 (?). It had a recess in the transom for the rudder. The rudder was actually a few inches inboard of the transom. I have also noticed that the recent fleet of blue-water racers have evolved so that the rudder(s) are located under the hull just forward of the transom.

What are the advantages/disadvantages of having the rudder under the hull? Is there a critical dimension from the keel to the rudder? Is there a disadvantage to having the rudder further aft of the keel?

I doubt that my modification will have any impact on how the tub sails. But, I am curious.

[SailDesign]

Quote: "What are the advantages/disadvantages of having the rudder under the hull? "

You won't break it if you back into something, and it can't ventilate if over-rotated. Those would be the two major advantages. Disadvantages? You have a new hole in the hull that wouldn't be there if you transom-mounted it.

If your modified rudder extends farther aft of its pivot point, (the vertical line drawn through the pintles) there will be more force opposing you on the tiller.

Tim Dunn

[SuperPiper]

The pintles are also moving outboard. The balance on the rudder should not change.

But here is an observation: the tiller gets heavy on this round little boat as she heels and loads up. She tracks OK with the rudder midships but it seems that the rudder is sharing some of the lateral resistance with the keel. This is not quite the same as weather helm is it?

[Hushinish]

my rudder is mounted on the stem, it works wonderfully.

[gonzo]

It will move the center of lateral resistance aft. This should decrease the effort on the tiller and reduce maneuverability. The extra space gives you the opportunity of balancing the rudder to take out some of the weather helm.

[Lew Morris]

I just fabricated a new kick-up rudder for my old McGregor Venture 21. Our launch facilities are sling only and the water at minus tide is only about twenty inches ... hence the kick-up feature.

Rather than just radius the corners off of a slab (like the original) I decided to build a foam/glass version using the NACA 0012 airfoil (another Boatdesign Forum topic) . I also added an inch of counter-balance to the leading edge (projecting forward of the pintle axis) at the same time.

WHAT A DIFFERENCE! Low speed maneuvering performance is incredible, and weather helm, and tiller pressure are (or seem) to be much reduced.

So Gonzo, if I relocate the rudder further aft, then the centre of lateral resistance will also move aft?

Does this mean that I may be able to order a roachy, full-battened mainsail without upsetting the balance of the boat?

Wow! I'm getting a bulge in my forsail. . .